Although Escherichia coli is usually a normal inhabitant of the gastrointestinal tract of man and animals, this organism is sometimes a cause of extraintestinal and intestinal infections. It is now clear that virulent E. coli possess a constellation of special genetic properties, i.e. specific adherence, hemolysin production, K1 antigen, and dulcitol fermentation, that are uncommon in fecal isolates. Little is kn wn of the genetic structure and function of these factors that appear to contribute to the virulence of these possible virulence factors by the newer techniques of genetics and molecular biology to ascertain whether they are plasmid-mediated or chrom somally-determined. Once the genetic loci are identified, we will begin a genetic analysis of the contribution of the various factors (singly or in combination) to the virulence of E. coli isolated from extraintestinal infections.